PALEOZOIC CORAL REEFS 247 



thopJiyllum, Stauria, Acervularia, PhiUipsastrcBa, Strombodes, Pachy- 

 phyllum, etcetera, and genera usually simple, bnt often with large indi- 

 viduals, as Zaplirentis, Streptelasma, Amplexus, Blothrophyllum, Cysti- 

 phyllum, Heliophyllum, etcetera, and Chcetetes. 



On the reefs of the present day the Hydroids are represented by 

 Millepora; the Alcyonaria by Heliopora and Tubipora, and the Madre- 

 poraria by the composite group of coral designated the Hexacoralla. 

 The same groups of reef-building organisms are represented in both the 

 Paleozoic and Eecent seas; in both they have the same facies as regards 

 growth-form ; in both their physiologic activity has resulted in the secre- 

 tion of large quantities of carbonate from the surrounding sea-water, 

 and in both submarine banks, known as coral reefs, have resulted. The 

 general similarity of the organisms and the similarity in the result of 

 their physiologic activity assuredly suggest similarity of conditions under 

 which the physiologic process took place. 



Certain Paleozoic reefs have been described in sufficient detail to give 

 additional information on the conditions under which they were formed. 

 Regarding the Silurian reefs of Gotland, we know that "on the flanks of 

 the reefs are found conglomerates and breccias of coral masses, such as 

 Halysites and Cystiplujllum, and crinoidal remains."^^ The matrix of 

 the Silurian reef exposed in Anschiitz' quarry, Cedarburg, Wisconsin, 

 "has the structure of a sandstone, by which name it is familiarly known." 



Grabau furnishes the following information on the Devonian reefs of 

 Wisconsin and New York: 



"The reefs in the vicinity of Alpena [Michigan] are best exposed in the 

 quarries opened in the Alpena limestone, which has a thickness of about 35 

 feet and is the middle member of the Hamilton or Traverse group in the 

 Thunder Bay region. Reefs occur in higher and, to some extent, in lower 

 strata of the group, but none of these are well exposed. 



"In outline the reef is roughly dome-shaped, with slopes sometimes as great 

 as 30 or 40 degrees. The height of the dome is equal to the thickness of the 

 limestone stratum — about 35 feet in this region — and the greatest diameter, 

 which is near the base, is perhaps several hundred feet. The chief reef- 

 builders represented are Favosites, Acervularia, and Stromatopora, which 

 form the main mass of the reef, while between them are found the smaller 

 corals and bryozoa, as well as brachiopods, crinoids, and a few other types 

 of organisms. There is an absence of stratification in the central reef mass, 

 the structure being exceedingly irregular. Between the corals and shells is 

 found a filling of coral sand, which generally consists of rather coarse frag- 

 ments with a predominance of crinoid joints. Solution and recrystallization 

 have not infrequently taken place, with the result that dog-tooth spar is of 

 common occurrence. 



"The coral heads are generally of large size; sometimes they are over- 



Grabau : Bull. Geol. Soc. America, vol. 14, p. 343. 



